Aniline is an important industrial chemical widely used as an intermediate in the preparation of dyes, photographic chemicals, agricultural chemicals, di-isocynates and rubber vulcanization accelerators.
A major commercial process for the preparation of aniline is based upon the reduction of nitrobenzene with hydrogen. This process involves the need to utilize large quantities of nitric acid as a nitrating agent and comparably large quantities of alkali to neutralize the waste acid purge, which generates significant environmental problems. Large amounts of wastewater containing high concentrations of salts are produced and must be carefully discarded. Moreover, the process of nitration of benzene is considered a highly hazardous operation.
For the reasons cited above, the manufacture of aniline by reaction of phenol with ammonia has become of major interest.
A number of processes for producing aniline by reaction of phenol with ammonia have been disclosed utilizing silica alumina catalysts, as well as zirconia-alumina, titania-alumina, zirconia-silica phosphate and tungsten oxide (See Japanese Patent Publication No. 23571/1967, for example).
U.S. Pat. No. 3,860,650 teaches the use of a precipitated alumina gel containing silica leached with boric acid or hydrochloric acid to reduce sodium below 1 percent by weight. Comparative examples of alumina containing silica made from natural clays which were leached with boric acid or hydrochloric acid to reduce sodium below 1 wt percent were shown not to be effective An additional example using an alumina containing only traces of sodium was tested. The alumina was Catapal, obtained now from the Vista Chemical Corporation, but originally Continental Oil Company. The alumina is prepared by precipitation as a fine crystalline powder from solution in organic alcohol. The initial activity of this catalyst expressed as a relative rate constant (k) was 0.2 compared to the Example 1 and Example 2 catalysts of choice which had rate constants (k) of 1.0 and 1.6, respectively. A fresh quantity of Catapal was leached with hydrochloric acid and retested Its initial activity expressed as a relative rate constant (k) was 0.2.
In U.S. Pat. No. 3,272,865, processes are described for the production of aniline by reaction of phenol with ammonia using catalysts selected from a group consisting of silica-alumina having from 10 percent to 20 percent silica, silica-alumina having 10 percent to 20 percent alumina, zirconia-alumina, titania-alumina, phosphoric acid and tungsten oxide. These catalysts although producing effluents from the reactor containing from 80 wt percent to 88 wt percent aniline, suffer from rapid activity declines arising from the formation of carbonaceous coatings. Interruption of the process in order to regenerate the catalyst and restore activity is required after less than 100 hours of operation.
The '865 patent also recites an example of a commercial alumina sample substituted for the silica-alumina of Example 1 and under the same process conditions. As shown in the '865 patent Example 3, with the alumina catalyst the reaction proceeds very slowly as is evidenced by the low formation of aniline. At a temperature of 475.degree. C., 45 percent of the phenol is converted to aniline. The inventor states that, "This example clearly shows that a gamma-alumina catalyst gives inferior conversions to phenol as compared to the catalysts of the instant invention."
In European Pat. No. Application No. 87907534.9 are disclosed processes for preparing aniline from a vapor phase reaction of phenol and ammonia, and for preparing the catalyst. As shown in Examples 2, 3, and 4, the preferred catalyst is a silica-alumina catalyst containing 9.9 wt percent silica having an alkali metal oxide content of less than 0.5 wt percent, said catalyst calcined at a temperature of 600.degree. C. -900.degree. C. followed by an acid treatment. The catalysts of European Application No. 87907534.9 show no loss in activity after 1000 hours of operation. This is compared to the silica-alumina catalysts, of low alkali content after acid treatment described in U.S. Pat. No. 3,860,650 which require regeneration after approximately 600 hours on stream.
Prior art which has addressed the desirability of developing a vapor phase process for production of aniline by reaction of phenol with ammonia have tended to focus on the use of silica-alumina catalysts. Such catalysts have in varying degrees been deficient in stability, may require regeneration after 40-600 hours or activity, and typically require elevated temperatures of 375.degree. C. for 98-99 percent conversion of phenol.